Recently, graphene, which has a single layer having a two-dimensional plane structure, exhibits excellent mechanical strength and thermal and chemical properties and can be produced into a thin film, has been highlighted and cases in which composite membranes are produced by transferring graphene to porous polymer supports are reported. Such a graphene-containing composite membrane can be used to separate oxygen from a mixed gas of nitrogen and oxygen to enrich oxygen or produce nitrogen, but it is disadvantageously inapplicable as a membrane for gas separation, because a graphene oxide membrane is formed by vacuum filtration and thus inevitably suffers from partial surface defects and has very low permeability and selectivity, in spite of the expectation of improvement in gas permeability and selectivity due to the thin film graphene (Patent Document 1).
In addition, application, to a chemical sensor or electrochemical double-layer capacitor, of a functionalized graphene-containing composite membrane having a coating layer coated with a functionalized graphene dispersion by vacuum filtration on a non-conductive porous polymer support is also well-known. However, applicability of this functionalized graphene-containing composite membrane to the membrane for gas separation is neither disclosed nor suggested. Even if the composite membrane is applied as a membrane for gas separation, the graphene oxide coating layer has a thickness of 50 to 500 nm and is too thick to use as a nano-scale thin film. For this reason, there is a limitation on improving permeability and selectivity of hydrogen by promoting permeation of hydrogen while preventing permeation of carbon dioxide, from a mixed gas of hydrogen and carbon dioxide (Patent Document 2).
In addition, research on incorporation of graphene oxide into a porous polymer support to improve permeability or selectivity to a certain gas mixture has been conducted, but a graphene oxide coating layer may be readily detached due to weak bonding force between the porous polymer support and the graphene oxide coating layer (Patent Document 3).
Meanwhile, a method of manufacturing a reduced graphene oxide film by reacting a graphene oxide film formed on a polyethylene terephthalate (PET) substrate at a temperature of 40° C. with a hydroiodic acid steam gas derived from a solution containing hydroiodic acid (HI) and acetic acid mixed in a ratio of 2:5 is well-known. However, it is not known that the composite membrane obtained by this method has a thickness of several nanometers and is thus applicable to a membrane for gas separation (Patent Document 4).
In addition, results of a variety of research on graphene/polymer nanocomposites containing graphene oxide or chemically or thermally reduced graphene oxide dispersed in various polymers are also well-known. However, a detailed description associated with effects of improving permeability of hydrogen and selectivity of hydrogen compared to carbon dioxide from the hydrogen/carbon dioxide mixed gas has not been reported yet. Meanwhile, it is not known that graphene oxide-based separation membranes developed to date are separation coating layers having a dense structure and are thus mainly applied as gas separation membranes for collecting carbon dioxide due to excellent permeability of carbon dioxide, and that the gas separation membranes are used for separating hydrogen from the hydrogen/carbon dioxide mixed gas due to limited methods of coating graphene oxide (Non-patent Document 1).